1. Field of the Invention
The present invention relates to a process for recording or reproducing optical information by a holography technique, and a process therefor.
2. Description of the Related Art
Holographic memories are disclosed in various documents, such as Japanese Patent Application Laid-Open No. 2006-267803. The holographic memory technique is described below with reference to FIGS. 9-11B, according to this patent document.
FIG. 9 is a development view for describing a conventional optical system for recording on a holographic memory disk (two-beam interference system). FIG. 10 is a development view for describing a conventional optical system for reproducing the information recorded on a holographic memory disk (two-beam interference system). This holographic memory disk 206 is a transmission type recording medium.
A conventional process for recording information on holographic memory disk 206 is described with reference to FIG. 9 and FIG. 11A. A light beam emitted from laser beam source 201 is converted to a parallel ray beam by collimator 202, and is introduced into beam splitter (BS) 203. This beam splitter (BS) 203 divides the introduced parallel ray beam into a transmitted light beam and a reflected light beam.
The transmitted light beam from beam splitter (BS) 203 penetrates through space light modulation element (SLM) 204 and objective lens 205, successively, and strikes holographic memory disk 206, as information light beam 212.
On the other hand, the reflected light beam from beam splitter (BS) 203 is further reflected by mirror 209. The reflected light beam from mirror 209 strikes holographic memory disk 206 as reference light beam 203.
FIG. 11A illustrates information light beam 212 and reference light beam 213 introduced into holographic memory disk 206. In recording layer 215 in holographic memory disk 206, information light beam 212 and reference light beam 213 interfere with each other, and an interference fringe 217, which is formed, is recorded as a refractive index distribution, to form a digital volume hologram. Multiple recordings can be made by changing the angle of incidence of projection of reference light beam 213 onto holographic memory disk 206, by adjusting the controlling mirror 209.
Next, a conventional process for reproducing the information recorded in holographic memory disk 206 is described below, with reference to FIG. 10 and FIG. 11B. A light beam emitted from laser beam source 201 is converted into a parallel ray beam by collimator 202 in the same manner as in the recording operation, and is introduced into beam splitter (BS) 203. This beam splitter (BS) 203 divides the introduced parallel ray beam into a transmitted light beam and a reflected light beam. The reflected light beam is further reflected by mirror 209, as reference light beam 213, to holographic memory disk 206.
In the information reproduction, the light beam transmitted through beam splitter (BS) 203 is intercepted by space-light modulation element (SLM) 204, not to reach holographic memory disk 206 through objective lens 205.
FIG. 11B illustrates reference light beam 213 introduced to holographic memory disk 206. Reproduced light beam 218 is generated by projection of reference light beam 213 to interference fringe 217 at the same angle of incidence as in the recording operation.
This reproduced light beam 218 is emitted from holographic memory disk 206 and travels through lens 210 to light-receiving element 211, to form an image thereon. The recorded information is reproduced from the output from light-receiving element 211. Further, reference light beam 213 is projected by controlling mirror 209 at another projection angle, which is the same as that of reference light beam 213 in the recording operation, to reproduce the intended information.
In the above conventional technique, the recording is made on a transmission type holographic memory disk 206. In information reproduction from transmission type holographic memory disk 206, reproduced light beam 218 is emitted from the face of holographic memory disk 206, reverse to the introduction of the reference light 213, and the reproduced light beam 218 is received by light-receiving element 211 of the optical system placed on the reverse side of holographic memory disk 206. Therefore, the apparatus inevitably has a large thickness (large height). This is disadvantageous.